1. Field of the Invention
The present invention relates to an imaging system for capturing an image of an object.
2. Description of the Related Art
Image pickup devices, in which photoelectric transducers are arranged one-dimensionally or two-dimensionally, include charge coupled devices (CCDs) and amplified MOS type image pickup devices. The amplified MOS type image pickup devices have the advantage of easy mounting in peripheral circuits and driving at low voltage. Accordingly, it is anticipated that the amplified MOS type image pickup devices will be utilized in the field of portable information devices.
FIG. 9 shows a schematic structure of an amplified MOS type image pickup device and the readout circuit of the amplified MOS type image pickup device.
Referring to FIG. 9, the amplified MOS type image pickup device includes pixels 100, vertical signal lines 107 (107a to 107e), and horizontal scanning lines 902 (902a to 902e). The pixels 100 will be described in detail below with reference to FIG. 10. The vertical signal lines 107 are used for transmitting outputs from the pixels 100 to a memory 909 for temporarily storing pixel outputs (signal storage). The horizontal scanning lines 902 are used for transmitting signals from a vertical scanning circuit (VSR) 914 to the pixels 100 in the N-th line (N denotes a natural number).
The structure in FIG. 9 includes switch metal oxide silicon (MOS) transistors 903 (903a to 903e), an output amplifier 904, the vertical scanning circuit 914, the memory 909 for temporarily storing pixel outputs, a horizontal scanning circuit (HSR) 910, and a gain controller 913. The switch MOS transistors 903 sequentially transfer the signals from the memory 909 to the output amplifier 904. The vertical scanning circuit 914 selects the pixels 100 (pixel lines) that output signal charge to the corresponding vertical signal line 107. The memory 909 temporarily stores the outputs from the pixels 100 in the N-th line. The horizontal scanning circuit (HSR) 910 sequentially outputs the outputs from the pixels 100 stored in the memory 909. The gain controller 913 controls the gain of the output amplifier 904.
FIG. 10 illustrates an equivalent circuit of a pixel 100 in the amplified MOS type image pickup device.
Referring to FIG. 10, the equivalent circuit includes a photodiode 101, a reset MOS transistor 102, a transfer MOS transistor 103, a source follower amplifier (amplification MOS transistor) 104, a line-selection MOS transistor 105, a floating diffusion region (hereinafter referred to as a FD region) 106, and the vertical signal line 107. The photodiode 101 converts an optical signal into signal charge (photoelectric conversion). The reset MOS transistor 102 resets the photodiode 101 and the FD region 106. The transfer MOS transistor 103 reads out the signal charge subjected to the photoelectric conversion in the photodiode 101. The source follower amplifier 104 performs voltage conversion for the readout signal charge and is connected to the FD region 106. The line-selection MOS transistor 105 supplies the output from the source follower amplifier 104 to the vertical signal line 107. Control signals φpres, φptx, and φpsel are applied to the gate electrodes of the reset MOS transistor 102, the transfer MOS transistor 103, and the line-selection MOS transistor 105, respectively.
The operation of the amplified MOS type image pickup device shown in FIG. 10 will now be described.
First, the reset MOS transistor 102 and the transfer MOS transistor 103 are turned on and the transfer MOS transistor 103 is then turned off to reset the photodiode 101. The photodiode 101 enters a storage state.
Next, turning off the reset MOS transistor 102 completes the resetting of the FD region 106. Turning on the line-selection MOS transistor 105 after a storage time “ts” elapsed activates the source follower amplifier 104 to turn on the transfer MOS transistor 103 in order to read out the signal charge subjected to the photoelectric conversion in the photodiode 101.
An efficient transfer method and a method of extending a dynamic range when the signal charge is transferred from the photodiode 101 to the vertical signal line 107 are disclosed in detail in Japanese Patent Laid-Open No. 11-261046 and Japanese Patent Laid-Open No. 2003-197890. In Japanese Patent Laid-Open No. 11-261046, for example, a technique for applying a voltage different from the power voltage to the gates of the reset MOS transistor 102 and the transfer MOS transistor 103 is disclosed. In Japanese Patent Laid-Open No. 2003-197890, for example, a technique for setting the threshold of the reset MOS transistor 102 for resetting the FD region 106 to a value smaller than the threshold of the source follower amplifier 104 is disclosed.
Owing to these techniques, the image qualities of digital cameras have been remarkably improved and are in the process of exceeding the image qualities of known silver films. Since detailed descriptions of such techniques are disclosed in the above patent documents, they are omitted herein.
Through the use of these techniques, it becomes possible to realize a digital camera that can extend the dynamic range and that can be accommodated to any sensitivity from International Organization for Standardization (ISO) 200, in terms of the sensitivity of a silver film, to ISO 1600 with one sensor by switching the gain of the output amplifier 904 after the photodiode 101 outputs the signal charge to the vertical signal line 107.
However, digital cameras that can capture images at low sensitivity, that is, at ISO 100, ISO 50, or ISO 025 in terms of the sensitivity of a silver film, have not been realized. This is because an increase in the signal to noise (S/N) ratio of image signals is desirable in a region having low ISO sensitivity.
In order to improve the S/N ratio of the image signals, it is necessary to sufficiently increase the amount of signal charge against the signals that cause noises. With this view, the photodiode 101 in one pixel is desirably designed so as to increase the area thereof. Accordingly, the areas of known photodiodes were increased and the widened photodiodes are used to produce sensors. In this case, there is a problem in that a large amount of signal charge subjected to the photoelectric conversion in the photodiode becomes saturated in the FD region 106.
Applying the technique disclosed in Japanese Patent Laid-Open No. 2003-197890 to increase a voltage from the power supply VDD, for resetting the FD region 106, (a reset voltage), results in a larger amount of signal charge, compared with known cases.
However, in the digital cameras having the increased reset voltage in the manner described above, there is a problem in that image capturing at high sensitivity causes so-called black crushing, in which the pixels close to black in the image of an object in a dark condition are roughly represented. It is presumed here that the voltage from the power supply VDD (reset voltage) is switched from 4.1V to 5V.
FIG. 11 is a log-log graph showing the relationship (photoelectric conversion characteristic) between the amount of light received by the photodiode 101 and the level of a signal output from the photodiode 101 to the vertical signal line 107.
Referring to FIG. 11, for example, when the ISO sensitivity is ISO 100, photoelectric conversion characteristic 1100, between the amount of light received by the photodiode 101 and the level of a signal received by the memory 909 through the corresponding vertical signal line 107, has a maximum value A1 of the amount of light and a maximum value S1 of the level of the signal. The signal received by the memory 909 in this manner is amplified by the output amplifier 904 shown in FIG. 9 and is converted into a digital signal of 10 bits or two bits by an analog-to-digital (A/D) converter (not shown).
When the ISO sensitivity is ISO 400, the amount of light received by the photodiode 101 is A2, which is one fourth of the amount of light received by the photodiode 101 at ISO 100. In addition, the memory 909 receives a signal at a signal level S2 corresponding to the amount of received light A2. The received signal is amplified fourfold by the output amplifier 904 and is converted into a digital signal of 10 bits or 12 bits by the A/D converter (not shown).
The black crushing referenced above will now be described with reference to FIG. 12.
FIG. 12 is a graph illustrating the enlarged photoelectric conversion characteristic of the photodiode 101 in a region, in FIG. 11, where both the amount of light received by the photodiode 101 and the level of a signal output from the photodiode 101 to the vertical signal line 107 are close to zero.
As seen from the photoelectric conversion characteristic 1100 shown in FIG. 12, the linearity of the relationship between the amount of received light and the signal level is reduced along with a decrease in the amount of received light (signal level). Although the relationship between the amount of light received by the photodiode 101 and the level of the signal output from the photodiode 101 are represented in the log-log graph in FIG. 12, a linear graph representing the relationship shows that the signal of the level corresponding to the amount of the signal charge is not transferred to the vertical signal line 107.
Such a phenomenon does not occur when the voltage from the power supply VDD (reset voltage) is set to 4.1V, but occurs when the voltage from the power supply VDD (reset voltage) is increased to 5V.
As described above, in related arts, it is difficult to properly perform the image capturing depending on the sensitivity of the photoelectric transducer and, furthermore, the image capturing cannot be properly performed when the sensitivity of the photoelectric transducer is too low or too high.